Why A repeater transmits and receives simultaneously on frequencies typically 600 kHz apart at 2 meters, so the duplexer must pass one frequency while rejecting the other by 80 dB or more just a fraction of a percent away. Only a resonant cavity, a tuned quarter-wave or half-wave section of coaxial or waveguide-like metal chamber, has the high Q and power handling needed to do that at 146 MHz. Several cavities are cascaded, often notch-tuned, to isolate the transmitter from the receiver front end.
Watch out Crystal filters have very high Q but are made for low-level IF frequencies in the kHz to tens of MHz range and cannot handle repeater transmitter power; DSP filters work on baseband or IF digital samples, not on the antenna feedline.
Repeater duplexer = big metal cans on the wall. High power plus razor-sharp VHF selectivity means cavity.